Showing papers on "Incubation published in 1999"

A single amino acid alteration (101L) introduced into murine PrP dramatically alters incubation time of transmissible spongiform encephalopathy.

Abstract: A mutation equivalent to P102L in the human PrP gene, associated with Gerstmann-Straussler syndrome (GSS), has been introduced into the murine PrP gene by gene targeting. Mice homozygous for this mutation (101LL) showed no spontaneous transmissible spongiform encephalopathy (TSE) disease, but had incubation times dramatically different from wild-type mice following inoculation with different TSE sources. Inoculation with GSS produced disease in 101LL mice in 288 days. Disease was transmitted from these mice to both wild-type (226 days) and 101LL mice (148 days). In contrast, 101LL mice infected with ME7 had prolonged incubation times (338 days) compared with wild-type mice (161 days). The 101L mutation does not, therefore, produce any spontaneous genetic disease in mice but significantly alters the incubation time of TSE infection. Additionally, a rapid TSE transmission was demonstrated despite extremely low levels of disease-associated PrP.

Physiological Characterization of Viable-but-Nonculturable Campylobacter jejuni Cells

Abstract: Campylobacter jejuni is a pathogenic, microaerophilic, gram-negative, mesophilic bacterium. Three strains isolated from humans with enteric campylobacteriosis were able to survive at high population levels (10(7) cells ml-1) as viable-but-nonculturable (VBNC) forms in microcosm water. The VBNC forms of the three C. jejuni strains were enumerated and characterized by using 5-cyano-2,3-ditolyl tetrazolium chloride-4',6-diamino-2-phenylindole staining. Cellular volume, adenylate energy charge, internal pH, intracellular potassium concentration, and membrane potential values were determined in stationary-phase cell suspensions after 48 h of culture on Columbia agar and after 1 to 30 days of incubation in microcosm water and compared. A notable increase in cell volume was observed with the VBNC state; the average cell volumes were 1.73 microliter mg of protein-1 for the culturable form and 10.96 microliter mg of protein-1 after 30 days of incubation in microcosm water. Both the internal potassium content and the membrane potential were significantly lower in the VBNC state than in the culturable state. Culturable cells were able to maintain a difference of 0.6 to 0.9 pH unit between the internal and external pH values; with VBNC cells this difference decreased progressively with time of incubation in microcosm water. Measurements of the cellular adenylate nucleotide concentrations revealed that the cells had a low adenylate energy charge (0.66 to 0.26) after 1 day of incubation in microcosm water, and AMP was the only nucleotide detected in the three strains after 30 days of incubation in microcosm water.

Males Feeding Females During Incubation. I. Required by Microclimate or Constrained by Nest Predation

Abstract: Nest attentiveness (percentage of time spent on the nest) during incubation represents a parent‐offspring conflict; incubating birds must balance a trade‐off between caring for embryos by staying on the nest versus caring for themselves by getting off the nest to forage. For species in which females are the sole incubator, males can potentially affect this trade‐off and increase nest attentiveness by feeding incubating females on the nest (incubation feeding). Increased nest attentiveness may be required when local microclimate conditions are harsh and thereby require greater incubation feeding (microclimate hypothesis). Alternatively, incubation feeding may be constrained by risk of attracting nest predators (nest predation hypothesis), which in turn may constrain female nest attentiveness because of energy limitation. We show that incubation feeding rates are much greater among cavity‐nesting than among coexisting open‐nesting birds. Under the microclimate hypothesis, the greater incubation fe...

Cold climates and the evolution of viviparity in reptiles: cold incubation temperatures produce poor-quality offspring in the lizard,Sceloporus virgatus

Abstract: Evolutionary origins of viviparity among the squamate reptiles are strongly associated with cold climates, and cold environmental temperatures are thought to be an important selective force behind the transition from egg-laying to live-bearing. In particular, the low nest temperatures associated with cold climate habitats are thought to be detrimental to the developing embryos or hatchlings of oviparous squamates, providing a selective advantage for the retention of developing eggs in utero , where the mother can provide warmer incubation temperatures for her eggs (by actively thermoregulating) than they would experience in a nest. However, it is not entirely clear what detrimental effects cold incubation temperatures may have on eggs and hatchlings, and what role these effects may play in favouring the evolution of viviparity. Previous workers have suggested that viviparity may be favoured in cold climates because cold incubation temperatures slow embryogenesis and delay hatching of the eggs, or because cold nest temperatures are lethal to developing eggs and reduce hatching success. However, incubation temperature has also been shown to have other, potentially long-term, effects on hatchling phenotypes, suggesting that cold climates may favour viviparity because cold incubation temperatures produce offspring of poor quality or low fitness. We experimentally incubated eggs of the oviparous phrynosomatid lizard, Sceloporus virgatus , at temperatures simulating nests in a warm (low elevation) habitat, as is typical for this species, and nests in a colder (high elevation) habitat, to determine the effects of cold incubation temperatures on embryonic development and hatchling phenotypes. Incubation at cold nest temperatures slowed embryonic development and reduced hatching success, but also affected many aspects of the hatchlings» phenotypes. Overall, the directions of these plastic responses indicated that cold-incubated hatchlings did indeed exhibit poorer quality phenotypes; they were smaller at hatching (in body length) and at 20 days of age (in length and mass), grew more slowly (in length and mass), had lower survival rates, and showed greater fluctuating asymmetry than their conspecifics that were incubated at warmer temperatures. Our findings suggest that cold nest temperatures are detrimental to S. virgatus , by delaying hatching of their eggs, reducing their hatching success, and by producing poorer quality offspring. These negative effects would likely provide a selective advantage for any mechanism through which these lizards could maintain warmer incubation temperatures in cold climates, including the evolution of prolonged egg retention and viviparity.

Heating nest–boxes reveals an energetic contraint on incubation behaviour in great tits, Parus major

Abstract: According to the ‘energetic–bottleneck’ hypothesis, incubation in birds is constrained by the availability of energy. Hence, uniparental incubators are predicted to respond to a change in energy supply by adjusting, positively or negatively, the time spent warming the clutch. Energetic constraints on incubation in the great tit ( Parus major ) were demonstrated by heating nests, so that the night‐time thermostatic component of daily energy expenditure in females was reduced by comparison with a control group. Birds in heated nests increased the time allocated to incubation during the day by 55 minutes, consistent with the predictions of the energetic–bottleneck hypothesis. Daily energy expenditure of all birds was inversely related to night–time ambient temperature, and did not differ between warmed and control birds on mild nights. When temperatures were low, however, escalation of daily costs was less for birds in heated nests. It is suggested that the balance of the energy budget may effect a proximate control on the constancy of incubation, with likely implications for reproductive success.

Effects of age on the generalization and incubation of memory in the F344 rat.

Abstract: Freezing (immobility) in the presence of aversive stimuli is a species-specific behavior that is used as an operational measure of fear. Conditioning of this response to discrete sensory stimuli and environmental context cues has been used as a tool to study the neuropsychology of memory dynamics and their development over the lifespan. Three age groups of F344 rats (3, 9, and 27 month) received tone-foot shock pairing (or tone only) in a distinctive chamber on two consecutive days. Separate subgroups of rats from each age group were then tested, at retention intervals of 1, 20, 40, or 60 days, for context-mediated fear in the environment in which they were trained, for generalization of the fear response to a novel chamber, and for fear of the tone. Beginning at day 20, the 27-month-old rats exhibited less freezing behavior than did younger rats when tested in the conditioning context. This age difference was a result of freezing behavior becoming progressively stronger with time in the two younger age groups, a phenomenon that has been referred to as memory incubation. Incubation of the contextual fear response was not detected in the old rats. In a novel context, all age groups exhibited significantly more freezing than did control animals. There was also pronounced incubation of this generalized freezing response, and the extent of incubation declined significantly with age. In the novel context, the freezing response to the tone was robust in all age groups and increased over time, in constant proportion to the degree of freezing elicited by the novel context itself, prior to tone onset. The fact that old animals are known to be relatively selectively impaired in forms of memory that depend on a functional hippocampus suggests a possible explanation for the reduced incubation effects seen in old rats; however, whether the increased expression of fear over time is mediated by a hippocampal-dependent memory consolidation process or whether it reflects a generalized increase in the gain of the circuitry mediating the fear response itself, remains to be determined.

Ecological genetics of seed germination regulation in Bromus tectorum L. : II. Reaction norms in response to a water stress gradient imposed during seed maturation.

Abstract: The probability that a seed will germinate depends on factors associated with genotype, maturation environment, post-maturation history, and germination environment. In this study, we examined the interaction among these sets of factors for 18 inbred lines from six populations of Bromus tectorum L., a winter annual grass that is an important weed in the semi-arid western United States. Seeds of this species are at least conditionally dormant at dispersal and become germinable through dry-afterripening under summer conditions. Populations and inbred lines of B. tectorum possess contrasting dormancy patterns. Seeds of each inbred line were produced in a greenhouse under one of three levels of maturation water stress, then subjected to immediate incubation under five incubation regimes or to dry storage at 20°C for 4 weeks, 12 weeks, or 1 year. Dry-stored seeds were subsequently placed in incubation at 20/30°C. Narrow-sense heritability estimates based on parent-offspring regressions for germination percentage of recently harvested seeds at each incubation temperature were high (0.518-0.993). Germination percentage increased with increasing water stress overall, but there were strong interactions with inbred line and incubation temperature. Inbred lines whose seeds were non-dormant over the full range of incubation temperatures when produced at low maturation water stress showed reaction norms characterized by little or no change as a function of increasing stress. For inbred lines whose dormancy status varied with incubation temperature, incubation treatments where seeds exhibited either very low or very high levels of dormancy showed the least change in response to maturation water stress. Inbred lines also varied in their pattern of dormancy loss during storage at 20°C, but maturation water stress had only a minor effect on this pattern. For fully afterripened seeds (1 year in storage at 20°C), inbred line and maturation water stress effects were no longer evident, indicating that differences in genotype and maturation environment function mainly to regulate dormancy and dormancy loss in B. tectorum, rather than to mediate response patterns of non-dormant seeds.

Incubation period of Creutzfeldt-Jakob disease in human growth hormone recipients in France.

Abstract: Objective: To estimate the statistical distribution of the incubation period of Creutzfeldt–Jakob disease (CJD) in human growth hormone (hGH) recipients in France. Background: Published papers suggest that the median incubation period of hGH-related CJD is approximately 15 years, but there are as yet no statistical data that support this assertion. Methods: Of the 1,361 hGH recipients who were included in this study, 55 had developed CJD at the time of the study. Individual data on hGH treatment history were available. Different mathematical models were used to estimate the statistical distribution of the incubation period. One main feature of the models was to take into account the occurrence of future CJD cases. Results: Models showed that the mean incubation period was 9 to 10 years, and the 95th percentile of the distribution was 15 to 16 years. Data and models indicated that the incubation period was significantly shorter in homozygotes at codon 129 of the prion protein gene than in heterozygotes. Conclusions: The short mean incubation period of CJD in French hGH recipients may be due to high infectivity in hormone lots. Estimates of the 95th percentile indicate that the number of hGH-related CJD cases may continue to increase in the coming years.

Augmentation of killing of Escherichia coli O157 by combinations of lactate, ethanol, and low-pH conditions.

Abstract: The acid tolerance of Escherichia coli O157:H7 strains can be overcome by addition of lactate, ethanol, or a combination of the two agents. Killing can be increased by as much as 4 log units in the first 5 min of incubation at pH 3 even for the most acid-tolerant isolates. Exponential-phase, habituated, and stationary-phase cells are all sensitive to incubation with lactate and ethanol. Killing correlates with disruption of the capacity for pH homeostasis. Habituated and stationary-phase cells can partially offset the effects of the lowering of cytoplasmic pH.

The influence of thermal and hydric environments on embryonic use of energy and nutrients, and hatchling traits, in the wall lizards (Podarcis muralis)

Abstract: Among (different) clutch variation in egg composition, influence of thermal and hydric environments on incubating eggs, embryonic use of energy and nutrients, and hatchling traits were studied in the wall lizard Podarcis muralis from a lowland population of Northern Spain When initial egg mass was kept constant, we found that some measured egg variables such as water, lipid-free organic material, ash, calcium and magnesium in egg contents, and ash and magnesium in eggshell, remained remarkably constant, whereas other variables differed considerably among clutches All viable eggs increased in mass over the course of incubation due to absorption of water, and mass gain during incubation was dependent on initial egg mass, temperature and substrate water potential Variations in the wet mass of hatchlings among treatments stemmed mainly from variations in water content Hatching success, embryonic use of energy and nutrients, and sex, size and mass of hatchlings were unaffected over a wide range of substrate moisture The incubation length decreased as temperature increased However, the effect of substrate moisture on duration of incubation varied with temperature The influence of incubation temperature on the snout-vent length of hatchlings, if present, was very weak Incubation temperatures did not affect the sex ratio and carcass dry mass of hatchlings, but significantly affected the tail length of hatchlings, with individuals from the highest temperature having the shortest tails The energy expenditure of embryogenesis during incubation remained remarkably constant among treatments, and energy reserves in the hatchling were largely dependent on allocation of energy materials in eggs A high incidence of dead-in-shell embryos occurred in eggs that were laid in June and, thereafter, when ambient temperatures were high Deformed hatchlings were distributed nearly equally among treatments

Validation of incubation duration as an index of the sex ratio of hatchling sea turtles

Abstract: One method of estimating the sex ratio of hatchling sea turtles is to use the incubation duration. Long and short durations imply low and high temperatures, respectively. In turtle species whose sex is determined by temperature, males are produced at low temperatures and females at high temperatures. This study assesses the validity of using incubation duration to estimate the sex ratio. Samples of hatchling loggerhead turtles ( Caretta caretta) were collected from nests with known incubation durations, and sex was ascertained by means of histology. The sex ratio of groups of nests determined by histology was compared with that predicted from previous relationships between incubation duration and sex ratio. For conditions causing relatively long or relatively short incubation durations, the sex ratio could be predicted with considerable accuracy. For conditions causing durations nearer to the pivotal duration (that which gives 50% of each sex), predictions could be off by 10%, depending on the distribution of incubation durations, but it was still possible to determine whether ratios were highly skewed or approximately balanced. Estimating sex ratios of hatchling sea turtles from incubation durations is simple, cheap, and can be used retrospectively. Resume : L'une des methodes d'estimation du rapport mâles : femelles chez des tortues marines fraichement ecloses

Response of the chitinolytic microbial community to chitin amendments of dune soils

Abstract: The dynamics of culturable chitin-degrading microorganisms were studied during a 16-week incubation of chitin-amended coastal dune soils that differed in acidity. Soil samples were incubated at normal (5% w/w) and high (15% w/w) moisture levels. More than half of the added chitin was decomposed within 4 weeks of incubation in most soils. This rapid degradation was most likely due to fast-growing chitinolytic fungi (mainly Mortierella spp. and Fusarium spp.) at both moisture levels, as dense hyphal networks of these fungi were observed during the first 4 weeks of incubation. Chitin N mineralization was inhibited by cycloheximide, and fast-growing fungal isolates were capable of rapid chitin decomposition in sterile sand, further suggesting that these fungi play an important role in initial chitin degradation. The strong increase in fast-growing fungi in chitin-amended dune soils was only detected by direct observation. Plate counts and microscopic quantification of stained hyphae failed to reveal such an increase. During the first part of the incubation, numbers of unicellular chitinolytic bacteria also increased, but their contribution to chitin degradation was indicated to be of minor importance. During prolonged incubation, colony forming units (CFU) of chitinolytic streptomycetes and/or slow-growing fungi increased strongly in several soils, especially at the 5% moisture level. Hence, the general trend observed was a succession from fast-growing fungi and unicellular bacteria to actinomycetes and slow-growing fungi. Yet, the composition of chitinolytic CFU over time differed strongly between chitin-amended dune soils, and also between the two moisture levels. These differences could not be attributed to pH, organic matter or initial microbial composition. The possible consequence of such unpredictable variation in microbial community composition for the use of chitin-amendments as a biocontrol measure is discussed.

Degradation of formulated and unformulated triticonazole fungicide in soil: effect of application rate

Abstract: Laboratory incubation studies were conducted to evaluate the influence of commercial formulation adjuvants and application rate over a range of 0.2–80 mg kg−1 on the dissipation of 14C-triticonazole systemic fungicide in a loamy clay soil, at 22 and 28°C, and 80% of water field capacity. Measurement of the balance of the 14C-residues after incubation at 22°C showed an increased resistance to desorption with time, as apparent desorption Kapp increased from 2 to 10 l kg−1 in a 133 d period. Decreased extractability of the residues with incubation time and formation of bound residues up to 20% showed that the overall availability of triticonazole decreased with incubation time. The addition of diluted doses of formulation adjuvants did not significantly affect the degradation and binding of the active ingredient. Mineralization of triticonazole was slow, with a high activation energy of 130 kJ mol−1. The persistence of triticonazole increased with application rate, as the first-order mineralization rates at 28°C decreased from 2×10−3 to 0.7×10−3 d−1 with amount applied increasing from 0.2 to 80 mg kg−1. However, the absolute amount of triticonazole mineralized increased with increasing concentration in the soil solution, and the slow mineralization was attributed to limited availability in the soil solution due to high sorption.

[The effect of the period of egg quiescence on the life cycle of Aedes aegypti (Linnaeus, 1762) (Diptera, Culicidae) under laboratory conditions].

Abstract: The influence of the period of egg quiescence on the life cycle of Aedes aegypti (Linnaeus, 1762) (Diptera, Culicidae) was studied under laboratory conditions in order to improve the management of vector control. The eggs are known to be the most resistant stages during development, allowing a long survival of the mosquitoes under unfavorable climatic conditions. The experiments were performed in a biological chamber kept at 28 +/- 1 degrees C temperature, with 80 +/- 5% relative humidity and 12 hours of photophase. Data about the influence of different periods of quiescence on eclosion, larval and pupal development and the developmental cycle are presented. We observed a highly significant effect of the period of quiescence on larval eclosion. The period of quiescence had no influence on the duration of larval or pupal incubation. Eggs originating from the same period of quiescence showed significantly different periods of incubation. The larvae emerged in groups defined by the period of incubation. This group effect was significant during the cycle. In 99.8% of the cycles the variation was determined by incubation.

Effects of PCB 126 on primary immune organ development in chicken embryos

Abstract: This experiment evaluated the immunotoxic effects of developmental exposure to a planar polychlorinated biphenyl (3,3',4,4',5-pentachlorobiphenyl; PCB 126) in chicken embryos. Previous investigations on the immunotoxic effects of PCBs and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in developing avian embryos were undertaken with embryos exposed only during the latter stages of incubation. To simulate exposure in embryos in the wild, chicken eggs were injected with PCB 126 (sunflower oil carrier) into the air cell before initiation of incubation. It was hypothesized that exposure to PCB 126 during the complete incubation period would decrease immune organ masses and lymphocyte numbers. Doses of PCB 126 ranged from 0.051 to 0.80 ng/g egg. Control groups consisted of carrier-injected and noninjected eggs. The thymus and bursa of Fabricius were removed and weighed on d 20 of incubation (1 d before hatch). The immune organs were homogenized, and viable lymphoid cells were counted using the trypan blue exclusion method. Probit analysis estimated the LD20 to be 0.21 ng/g and the LD50 to be 1.01 ng/g. Thymus mass dropped sharply between 0.13 and 0.32 ng/g, and lymphoid cell numbers in the thymus fell sharply between 0.051 and 0.13 ng/g. Bursa mass began to decrease at the lowest dose of 0.051 ng/g and reached a minimum at 0.32 ng/g. The number of viable cells decreased slightly at 0.051 ng/g and reached a minimum at the 0.13- and 0.32-ng/g doses. In general, lymphoid cell numbers were more sensitive to PCB 126 than organ masses, and the bursa tended to be more sensitive than the thymus. Doses necessary to reduce the number of viable lymphoid cells in the thymus and bursa were at least one order of magnitude lower with full-term incubation as compared to exposure only during later stages of incubation.

Effects of Temperature on Growth and Efficiency of Yolk Utilisation in Eggs and Pre-feeding Larval Stages of Atlantic Salmon

Abstract: Three embryonic stages of Atlantic salmon (Salmo salar L.) were subjected to eight constant incubation temperatures (4, 7, 10, 12, 14, 16, 19 and 22 °C) exceeding the range usually experienced in natural conditions. A change in thermal tolerance during the embryonic and larval development was registered: pre-hatching stages showed an upper thermal limit at about 16 °C, while hatched larvae survived until 22 °C. Temperature significantly affects developmental rate, resulting in a faster development and, consequently, lower yolk weight percentage at higher temperatures. We found positive relationships between incubation temperature and body size (length and weight) in the less developed stages, in which some yolk remained, but size decreased at increasing temperatures when yolk was completely exhausted.

Incubation of OKP cells in low-K+ media increases NHE3 activity after early decrease in intracellular pH.

Abstract: Chronic hypokalemia increases the activity of proximal tubule apical membrane Na+/H+antiporter NHE3. The present study examined the effect of the incubation of OKP cells (an opossum kidney, clone P...

Glucocorticoids enhance acid activation of the Na+/H+ exchanger 3 (NHE3)

Abstract: In the absence of exogenous glucocorticoids, decreasing media pH (from 7.4 to 6.8) for 24 hours increased the Na+/H+ exchanger 3 (NHE3) activity in opossum kidney (OKP) cells. 10–7 M and 10–8 M hydrocortisone increased NHE3 activity, and in their presence, acid incubation further increased NHE3 activity. Hydrocortisone (10–9 M) had no effect on NHE3 activity, but in its presence, the effect of acid incubation on NHE3 activity increased twofold. Aldosterone (10–8 M) had no effect. In the absence of hydrocortisone, acid incubation increased NHE3 protein abundance by 47%; in the presence of 10–9 M hydrocortisone, acid incubation increased NHE3 protein abundance by 132%. The increase in NHE3 protein abundance was dependent on protein synthesis. However, 10–9 M hydrocortisone did not modify the effect of acid incubation to cause a twofold increase in NHE3 mRNA abundance. In the absence of protein synthesis, 10–9 M hydrocortisone did potentiate an effect of acid on NHE3 activity, which was due to trafficking of NHE3 to the apical membrane. These results suggest that glucocorticoids and acid interact synergistically at the level of NHE3 translation and trafficking.

Sex differences in optimal incubation temperatures in a scincid lizard species.

Abstract: Most theoretical models for the evolution of temperature-dependent sex determination (TSD) rely upon differential fitness of male and female offspring incubated under different thermal regimes. However, there are few convincing data on this topic. We studied incubation effects in a lizard species (Bassiana duperreyi, Scincidae) with genotypic sex determination, so that we could separate effects due to incubation temperatures from those due to offspring gender. We incubated eggs under two different fluctuating-temperature regimes that simulated hot and cold natural nest-sites. The effects of our incubation treatments on phenotypes of the hatchling lizards (morphology and locomotor performance) differed between the sexes. Females emerging from eggs exposed to the “hot nest” treatment (diel cycling, 23–31°C) were larger, and ran faster, than did their sisters from the “cold nest” treatment (16–24°C). Males showed a smaller and less consistent phenotypic response than females. These incubation-induced responses were relatively stable during the first few weeks of life post-hatching, at least in captive lizards maintained under laboratory conditions. These kinds of sex differences in the phenotypic responses of hatchling reptiles to incubation conditions provide a plausible basis for the evolution of temperature-dependent sex determination in reptiles.

Incubation temperature and sex affect mass and energy reserves of hatchling snapping turtles, Chelydra serpentina

Abstract: Temperature during embryonic development irreversibly determines gonadal sex in many oviparous reptiles. Although embryonic temperature also influences a number of other traits in these species, it is unclear whether such effects are primarily due to incubation temperature or to gonadal sex. Here we dissociated these normally confounding effects via hormonal manipulations of snapping turtle embryos (Chelydra serpentina), a species with temperature-dependent sex determination. We then ascertained temperature and sex effects on residual yolk mass, abdominal fat body mass, and total mass in neonates shortly after hatching or after one month without feeding. Yolk mass was initially affected by incubation temperature, clutch identity. and their interaction, but not gonadal sex. Yolk mass was not influenced by any of these variables 30 d after hateching. Fat mass was initially affected by temperature and clutch, but not sex. Gonadal sex did, however, affect fat mass at 30 d of age as did temperature and clutch. Incubation temperature affected total mass in a complex manner. There were significant clutch by temperature interactions initially, but no main effect of temperature. These effects changed so that the temperature effect was significant but interaction effects were not detected at 30 d of age. Repeated measures analysis of total mass after hatching and at 30 d of age indicated that turtles from lower temperatures lost more mass than turtles from higher incubation temperatures. and that females lost more mass than males. These data indicate that some hatchling traits were influenced directly by incubation temperature, whereas others were independently affected by gonadal sex. These results are consistent with a leading hypothesis for the evolution of temperature-dependent sex determination.